1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor and a manufacturing method thereof.
In this specification, a semiconductor device means all types of devices which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and electronic equipment are all semiconductor devices.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of approximately several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and thin film transistors that are used as switching elements in image display devices are, in particular, urgently developed. Indium oxide, which is an example of metal oxides, is used as a transparent electrode material necessary for liquid crystal displays or the like.
Some metal oxides have semiconductor characteristics. Examples of the metal oxides having semiconductor characteristics are tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. Thin film transistors in which a channel formation region is formed using such a metal oxide having semiconductor characteristics are already known (Patent Documents 1 to 4, Non-Patent Document 1).
As the metal oxides, not only single-component oxides but also multi-component oxides are known. For example, InGaO3(ZnO)m (m: natural number) having a homologous series is known as a multi-component oxide semiconductor including In, Ga, and Zn (also referred to as an In—Ga—Zn-based oxide) (Non-Patent Documents 2 to 4).
Furthermore, it is confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide is applicable to a channel layer of a thin film transistor (Patent Document 5, Non-Patent Documents 5 and 6).